Among the institutions that could be downgraded are Bank of America Corp , Citigroup Inc and Morgan Stanley , said Baylor Lancaster, an analyst at CreditSights Inc.
Spokesmen for the three banks declined to comment.
Some European banks could also be affected. On November 9, S&P downgraded its scores for the health of the banking industries in a number of countries, including Denmark, Sweden, Finland and the Netherlands.
The updates in ratings are part of a major overhaul of S&P's methods for scoring the creditworthiness of some 750 banking groups.
The agency, the subject of intense criticism because its positive ratings for mortgage-backed securities played a major role in inflating the U.S. housing bubble, has been working on the changes for more than a year.
The updates are part of a broad push by S&P to improve its products and repair its reputation as its parent, McGraw-Hill Cos Inc , divides itself into two publicly traded companies.